It is known that the manufacture of prestressed concrete poles involves the casting and hardening of concrete in molds with special requirements for the operations of positioning, stressing and loosening the mold. To date, there have been many commercial articles made from prestressed concrete materials. These articles include tubular pipes, beams, support columns, and poles, which may be reinforced by steel strands embedded in the concrete or by rods, bars, and/or tubes for reinforcement located in the concrete. A large number of prestressed concrete articles have been manufactured and it is always desirable to provide means which makes manufacturing of these articles less expensive as well as easier to accomplish.
Prestressed concrete lamp poles are cast in a mold. The empty mold is assembled with a number of separate pieces bolted together, and in order to prestress a concrete pole, an axial compressive load is placed on the mold. The axial load is transmitted through the mold by prestressing strands which extend through the mold and are attached at either end of the mold. After prestressing the strands in tension, the mold is pumped full of concrete and rotated to centrifugally consolidate the concrete. Once the mold is full, it is generally conveyed through a heated chamber to accelerate the curing of the concrete. The prestressing load is then ready to be transferred from the mold to the pole and the mold can be removed from the finished product.
Typical molds used to cast concrete poles are solid steel molds. Depending upon the intricacy or the ornamentality of the pole to be cast, the mold is composed of separate segments corresponding to each distinct pattern located on the pole. Solid steel molds are composed of two or more pieces in which the pattern to be cast is machined into the interior surface of the mold pieces such that when the mold pieces are joined together a hollow cavity exists corresponding to the pattern of the pole.
A number of patterns can be incorporated into a single pole, however, it is also common for a single pattern to dominate a pole structure. A very popular pattern for a concrete lamp pole is a fluted pattern. Generally, a fluted pattern dominates the overall pattern of a lamp pole. Sometimes in a fluted pole very ornate patterns are located at the base and top ends of the pole which include circumferentially extending extensions and depressions from the general surface of the pole.
Many concrete pole molds are made of segments to be interchangeable so that the designs of the poles can be varied by changing one or more of the segments without having to reconstruct the entire mold. Pole molds are also constructed in circumferential sectors, for example, three 120 degree sectors, which makes it easier for the mold to be stripped from a cast pole. The particulars of this type of mold can be understood from applicant's patent application Ser. No. 07/683,303, filed Apr. 10, 1991 now U.S. Pat. No. 5,178,887, and incorporated by reference as if fully set forth herein.
When a pole has been cast and the prestressing load transferred from the mold to the pole, it has been experienced that the length of the standard, smooth surface, straight tapered pole, shortens as much as one-half of an inch, depending upon the pole length, before the pole has fully cured. With this type of pole, the shortening causes no damage and presents no problem, however, it has been experienced that with ornate poles, with undercut areas, when the prestressing load is transferred to the pole while still in the mold, the decorative configurations are severely damaged by pole shrinkage. The mold elongates as compressive stress is relieved and the concrete pole shortens as the prestressing load is applied. The differential motion can cause the pole to stick in the mold and shear off raised decorative elements.
Thus, there exists a need for mold stripping equipment which allows the mold to be removed before the prestressed tension is applied to the pole and thereby eliminate any damage due to shrinkage.